In the United States a standard has been proposed for digitally encoded high definition television signals. A portion of this standard is essentially the same as the MPEG-2 standard, proposed by the Moving Picture Experts Group (MPEG) of the International Organization for Standardization (ISO). The standard is described in a International Standard (IS) publication entitled, "Information Technology--Generic Coding of Moving Pictures and Associated Audio, Recommendation H.626", ISO/IEC 13818-2: 1995 (E) which is available from the ISO and which is hereby incorporated by reference for its teaching on the MPEG-2 digital video coding standard.
The MPEG-2 standard is actually several different standards. In MPEG-2, several different profiles are defined, each corresponding to a different level of complexity of the encoded image. For each profile, different levels are defined, each level corresponding to a different image resolution. One of the MPEG-2 standards, known as Main Profile, Main Level (MP@ML) is intended for coding video signals conforming to existing television standards (i.e., NTSC and PAL). Another standard, known as Main Profile, High Level (MP@HL) is intended for coding high-definition television images.
Images encoded according to the MP@HL standard may have as many as 1,152 active lines per image frame and 1,920 pixels per line. The MP@ML standard, on the other hand, defines a maximum picture size of 720 pixels per line and 567 lines per frame. The high definition television standard proposed for HDTV encoding in the United States is a subset of the MP@HL standard, having as many as 1,080 lines per frame, 1,920 pixels per line and a maximum frame rate, for this frame size, of 30 frames per second.
The MPEG-2 standard defines a complex syntax which contains a mixture of data and control information. Some of this control information is used to enable signals having several different formats to be covered by the standard. These formats define images, having differing numbers of picture elements (pixels) per line, differing numbers of lines per frame or field and differing numbers of frames or fields per second. In addition, the basic syntax of the MPEG-2 Main Profile defines the compressed MPEG-2 bit stream representing a sequence of images in six layers, the sequence layer, the group of pictures layer, the picture layer, the slice layer, the macroblock layer, and the block layer. Each of these layers except the block layer is introduced with control information. Finally, other control information, also known as side information, (e.g. frame type, macroblock pattern, image motion vectors, coefficient zig-zag patterns and dequantisation information) are interspersed throughout the coded bit stream.
An implementation of a HDTV system should be compatible with existing systems such as NTSC and PAL. Accordingly, to effectively receive the digital images, an HDTV decoder should be able to generate a picture corresponding to the MP@HL standard or the MP@ML standard to provide compatibility (i.e., through a set-top decoder) with existing receivers. For example, the HDTV decoder should be able to generate a picture corresponding to the MP@HL standard from a MP@ML encoded signal or a picture corresponding to the MP@ML standard from a MP@HL encoded signal.
In addition, the use of new receivers having a high definition video monitor which can display an image generated from a MP@HL signal may be cost prohibitive for certain customers. Accordingly, an HDTV decoder which can generate a picture corresponding to the MP@HL standard or the MP@ML standard will allow the use of newer, lower cost receivers which have video monitors that provide a higher resolution than MP@ML but less than MP@HL. A television set containing one of these monitors should still be able to display an image generated from a MP@HL signal albeit at a lower resolution.
Using existing techniques, a decoder may be implemented using an interpolation circuit to interpolate or decimate signals in the pixel domain to reduce the resolution of the image from MP@HL to the MP@ML. To process an MP@HL encoded image to produce an MP@ML signal by these conventional methods, the MP@H encoded image is converted from the spatial frequency domain to the pixel domain, interpolated to produce a reduced resolution image, and then the interpolated image is converted back to the spatial frequency domain. This type of processing would be contrary to the implementation of a decoder in a consumer television receiver in which cost is a major factor, Additional circuitry would be required to decode, interpolate or decimate, and then code the signal. Furthermore, the decoding and encoding operations may introduce undesired artifacts into the image that is represented by the reduced-resolution coded signal.
Another alternative is to use a decoder which selects a portion of the incoming HDTV bit stream, before decoding, as shown in SDTV RECEIVERS WITH HDTV DECODING CAPABILITY, by Jill Boyce et al., dated February 1995 and presented at the ACATS Technical Subgroup Meeting, May 18, 1995. The decoder disclosed in the paper utilizes a number of techniques in an attempt to reduce the cost and complexity for a standard television receiver. A pre-parser examines the incoming bit-stream and discards coding elements, DCT coefficients, of less importance. These elements include the high frequency DCT coefficients. The coding elements are then converted from the spatial frequency domain to pixels in the pixel domain. The pixels are then down-sampled to produce pixel for a lower resolution image.
There are other systems which have been proposed which utilizes other transform techniques to encode an image. For example, Fourier Transforms have been used to encoded images in the spatial frequency domain. One such system is shown at page 18.3 of the Television Engineering Handbook by K. Blair Benson, dated 1992, which is incorporated herein by reference for its teachings on digitally encoding images. This system utilizes Fourier Transforms to encode the image. As in the MPEG standard, this system could be used to encode images at different resolutions. Thus, the need for the conversion between higher and lower resolution images to provided compatibility between different receivers is also required for these systems.